The application of relatively small quantities of liquid to a drum surface or to the surface of a web or other substrate, e.g. a surface on a drum or guided around a drum, is important in a variety of fields.
For example, in printing, the liquid may be a color-carrying medium, e.g. a printing ink or dyestuff, which must be applied to the surface of a fabric web or to some other flexible surface. In printing applications, moreover, it frequently is desirable to apply a liquid, e.g. a masking liquid or an information-carrying liquid, to a drum which can then be used to transfer the liquid, to provide a graphic result or for some other purpose.
In yet other fields, e.g. in the coating of materials, it is desirable to transfer liquid from a bulk supply in more or less film form to a substrate directly or via a drum. In still other fields, the transfer of a thin liquid layer to a web or substrate may be desirable to effect a reaction, bring about a change in a physical state or simply as a temporary measure utilizing the liquid in some process application, or to coat the substrate permanently or temporarily.
It is not uncommon where the liquid must be transferred to a surface, to provide a liquid-containing trough, a liquid-applying roller partially immersed in this trough for entraining the liquid to the surface, and a doctor blade or the like which in part controls the thickness of the layer applied to the surface.
It is known in connection with such transfer rollers, moreover, to utilize grooved or otherwise controlled application rollers, especially where more viscous liquids are to be transferred, i.e. the material to be transferred has more or less a pasty character.
It is also known to smooth out the layer on the surface of such roller with a doctor blade, to remove excess liquid from the surface of the roller with a stripping blade and to operate, in the latter cases, with relatively large quantities of liquid which uncontrollably are in contact with the atmosphere are circulated within the container in which the roller is mounted.
More specifically, mention may be made of the two major problems hitherto encountered when an applying roller was used to transfer liquid to a surface moving past this roller from a trough in which the roller was located. In those cases in which a relatively wide trough was used, as was common, the liquid-applying roller could entrain a film of air into the liquid in the trough and because of the stirring action and the presence of this air, could cause the liquid to foam to the detriment of uniform layer application. When the trough was used merely to apply the layer from the bottom of the trough to the surface, for example, so that entrainment of an air film into the liquid in the trough was not as significant a problem, difficulties were encountered in effectively adjusting the gap through which the liquid was transferred to the surface and again uniformity of the film was adversely affected or the film had to be much thicker than was desired.